1. Technical Field
This invention relates to a function testing system for determining whether a printed circuit board having electronic parts is defective or not.
2. Background Information
According to conventional function testing systems for printed boards, a signal is applied to an input terminal of a circuit board being tested and a value of a resulting signal appearing at an output terminal of the circuit board is measured by means of a digital voltmeter, oscilloscope or other measuring instrument. The operation of the circuit board is then compared with an operational simulation of the circuit board, which is based on the design drawing, to determine whether the circuit board is defective. To automate this type of function testing system, it is necessary to characterize the operational simulation in software and store the software in a computer. However, such a system is quite complicated. Thus, a need exists for a simplified function testing system.
A number of systems disclosed in the art have specific operational problems which render the systems unsatisfactory. According to the system disclosed in U.S. Pat. No. 4,510,572, the signature of a digital circuit previously confirmed as being non-defective is stored in a memory. This signature is compared with the signature of a test digital circuit to determine whether the test digital circuit is defective. Since the system utilizes the signature in digital form as its comparison parameter for testing, the system is inapplicable to testing circuit boards providing an analog signal. Although it is possible to convert the analog signal into a digital signal using an A/D converter or the like, if the analog signal from the circuit board has even small fluctuations falling within a reasonable range, the digital signal obtained by conversion will have large fluctuations. This will produce an erroneous indication that the circuit is defective when it is not. Thus, the system of this patent is inferior because it can only accurately test digital circuits.
Other systems generally called in-circuit testers are disclosed in U.S. Pat. Nos. 4,686,627 and 4,688,628. With an in-circuit tester, every electronic part mounted on a circuit board is tested, and then the circuit board is judged by synthesizing the individual test results of each electronic part. Therefore, a large number of points on the circuit board must be laboriously measured in a time consuming manner.
Further systems, as disclosed in U.S. Pat. Nos. 4,493,045 and 4,652,814, are intended to solve the foregoing problems of the in-circuit tester. The systems generate test vectors to effectively shorten the measurement time. However, such systems have proven to be unsatisfactory.